Insights into the regulation of normal and depressed hematopoiesis have been provided by the study of patients with bone marrow failure and their tissues. Laboratory studies have focused on three aspects of bone marrow depression: 1) the cell biology of hematopoietic progenitors; 2) immunological aspects of normal and aberrant bone marrow regulation, especially the role of lymphokines in stimulation and suppression of bone marrow cell growth; 3) the role of viruses as etiologic agents in aplastic anemia. In an effort to improve the treatment of patients, the Clinical Hematology Branch is directing a large, multi-center trial of anti-thymocyte globulin in aplastic anemia and related bone marrow disorders. More recently we have investigated the use of Acyclovir, an anti-viral agent, in patients with aplastic anemia.